A microalloyed steel now has been widely used as a raw material of a machine structural part. For example, in a microalloyed steel having vanadium (V) added therein, V which is added in a trace amount forms a minute carbide and contributes to realization of high strength of the part.
For example, for crankshafts as the machine structural part, for the purpose of increasing wear resistance or fatigue strength, a nitrocarburizing treatment such as a salt bath nitriding treatment, a gas nitrocarburizing treatment, a plasma nitriding treatment, etc. may be possibly applied after the foregoing machining. In general, it is known that the nitrocarburizing treatment is low in the generation amount of a strain after the heat treatment. But, at the time of manufacturing a crankshaft, for the purpose of further enhancing the precision, straightening process for correcting bending to be inevitably caused due to the nitrocarburizing treatment is applied. However, there were involved problems that the microalloyed steel to which a nitrocarburizing treatment has been applied is lower in straightening ability than a thermal refined steel to which a nitrocarburizing treatment has been applied and that the fatigue strength is easily lowered due to influences of a stress strain generated on the surface at the time of straightening process. Then, in order to enhance the straightening ability and fatigue strength of a microalloyed steel, various proposals have been made (see, for example, Patent Document 1)    Patent Document 1: JP-A-2007-231302